1. Field of the Invention
The present invention relates to a speed-change control apparatus for an automatic transmission that is installed in a motor vehicle and, more specifically, to a speed-change control apparatus for controlling both engine torque and hydraulic pressure for changing speed stages and, in particular, to a speed-change control apparatus which provides a multiple speed-change operation wherein, during a certain speed-change operation, a command to establish another speed stage is issued.
2. Description of the Related Art
U.S. Pat. No. 4,688,450 discloses a speed-change control apparatus for changing engine torque during a speed-change operation. This speed-change control apparatus relates to speed-change control wherein during a certain (first) speed-change operation, a command to establish another (second) speed-change stage is issued. In this speed-change control apparatus, if a command to perform the second speed-change operation is issued prior to termination of the first speed-change operation, the engine torque control is suspended immediately as to both the first and second speed-change operations.
For example, as shown in FIG. 11, during an up-shift operation (e.g. from the first speed to the second speed), as the speed-change operation proceeds through engagement and disengagement of frictional engagement elements, the rotational speed N.sub.T of the input shaft continues to rise with a low-speed gear ratio in a torque phase, whereas the rotational speed of the input shaft falls toward a high-speed gear ratio in an inertia phase (see the line of alternating short and long dashes in FIG. 11). In this state, if it is determined that the inertia phase has been reached based on detection of a change in rotational speed of the input shaft, a control routine is performed to reduce the engine torque T.sub.E by the predetermined amount (T.sub.C). Then, if a command to perform a down-shift operation (e.g. from the second speed to the first speed) is issued, the aforementioned engine torque control is suspended immediately. Then, a command to set the engine torque back to an intrinsic torque T.sub.E is issued.
In the aforementioned speed-change control apparatus, upon issuance of a command to perform the second speed-change (down-shift) operation, the hydraulic pressure that is on the engagement side during the first speed-change (up-shift) operation is reduced immediately, so that a change in rotational speed (a rise in rotational speed of the input shaft) is generated through the second speed-change (down-shift) operation. On the other hand, as described above, a command to suspend the engine torque control is also issued immediately in response to the command to perform the second speed-change (down-shift) operation. Therefore, the rotational speed of the input shaft tends to rise. In circumstances where the engine torque may change abruptly, the hydraulic pressure control needs to be performed during the second speed-change (down-shift) operation. However, the characteristic of the feedback control of the aforementioned hydraulic pressure for change in rotational speed of the input shaft is unsuitable. That is, as indicated by a dotted line in FIG. 11, the rotational speed N.sub.T of the input shaft rises abruptly and tends to overshoot.